Q: In a closed system, the following concentrations are measured: $$[A] = 0.2 M$$, $$[B] = 0.3 M$$, $$[C] = 0.1 M$$, $$[D] = 0.4 M $$for the reaction $$A + B \rightleftharpoons C + D. If$$ $$K_c = 0.67$$, what is the value of $$Q_c$$ and in which direction will the reaction proceed?

$$Q_c = \frac{[C][D]}{[A][B]} = \frac{0.1 \times 0.4}{0.2 \times 0.3} = 0.67$$; the system is at equilibrium.

Q: For the reaction $$2A \rightleftharpoons B + C$$, if $$K_c = 1.5$$ and the initial concentrations are $$[A] = 1.0 M$$, $$[B] = 0.5 M$$, $$[C] = 0.5 M$$, what is $$Q_c$$ and what will happen?

$$Q_c = \frac{[B][C]}{[A]^2} = \frac{0.5 \times 0.5}{(1.0)^2$} = 0.25$$; reaction will shift right.

Question 1

Based on the image, which of the following statements best describes the relationship between $$Q_c$$ and $$K_c$$ and the direction in which a reaction will proceed?

Accepted Answer

If $$Q_c \u003c K_c$$, the reaction will proceed to the right (toward products) until equilibrium is reached.

Question 2

A reaction mixture has $$Q_c \u003e K_c. $$According to the image, what will happen to the concentrations of products and reactants as the system moves toward equilibrium?

Accepted Answer

The concentration of products will decrease and the concentration of reactants will increase.

Question 3

If $$Q_c = K_c$$ for a chemical reaction, what does this indicate about the state of the system?

Accepted Answer

The system is at equilibrium and no net change will occur in the concentrations of reactants and products.

Question 4

For the reaction $$aA + bB \rightleftharpoons cC + dD$$, the reaction quotient $$Q_c is $$given by which of the following expressions?

Accepted Answer

$$Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$$

Question 5

A student observes that $$Q_c is $$much less than $$K_c$$ for a reaction mixture. What should the student expect to happen as the system approaches equilibrium?

Accepted Answer

The reaction will shift to the right, forming more products.

Question 6

In a closed system, the following concentrations are measured: $$[A] = 0.2 M$$, $$[B] = 0.3 M$$, $$[C] = 0.1 M$$, $$[D] = 0.4 M $$for the reaction $$A + B \rightleftharpoons C + D. If$$ $$K_c = 0.67$$, what is the value of $$Q_c$$ and in which direction will the reaction proceed?

Accepted Answer

$$Q_c = \frac{[C][D]}{[A][B]} = \frac{0.1 	imes 0.4}{0.2 	imes 0.3} = 0.67$$; the system is at equilibrium.

Question 7

For the reaction $$2A \rightleftharpoons B + C$$, if $$K_c = 1.5$$ and the initial concentrations are $$[A] = 1.0 M$$, $$[B] = 0.5 M$$, $$[C] = 0.5 M$$, what is $$Q_c$$ and what will happen?

Accepted Answer

$$Q_c = \frac{[B][C]}{[A]^2} = \frac{0.5 	imes 0.5}{(1.0)^2$} = 0.25$$; reaction will shift right.

Question 8

A chemist adds more reactant to a system at equilibrium. According to the reaction quotient and the image, what will happen to $$Q_c$$ and the direction of the reaction?

Accepted Answer

$$Q_c$$ will decrease and the reaction will shift to the right to form more products.

Question 9

If a reaction mixture initially has only products and no reactants, what can be said about $$Q_c$$ and the direction the reaction will proceed?

Accepted Answer

$$Q_c$$ will be very large and the reaction will shift to the left to form more reactants.

Question 10

Which of the following best explains why $$Q_c$$ changes as a reaction proceeds toward equilibrium?

Accepted Answer

As the concentrations of reactants and products change, the value of $$Q_c$$ changes until it equals $$K_c at $$equilibrium.

Chemical Equilibrium: Reaction Quotient, Equilibrium Calculations, and Le Chatelier's Principle

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